The general goal of this project is to elucidate the mechanisms responsible for the evolution and maintenance of plasmids, phage and transposons in bacterial populations and evaluate the role of these accessory genetic elements and infectious gene transfer in the adaptation and evolution of bacteria. The proposed investigations have three parts. (1) Plasmid Studies: We will examine the selection pressures responsible for; i) maintaining genes and transposons on plasmids, rather than chromosomes, and ii) maintaining multiple plasmids in single bacteria. As part of this research we will determine the effects of antibiotics on the rates of conjugative R-plasmid transfer and evaluate the selection pressures and genetic mechanisms responsible for increasing the number of antibiotic resistance genes and transposons on single plasmids. (2) Recombination Studies: In an effort to evaluate the role of the horizontal transfer of chromosomal genes in the adaptation and evolution of bacteria we will; i) determine the effects of recombination of the rate of evolution of prototropy and expansion of fermentation capabilities in E. coli; ii) model and examine the kinetics of natural transformation in Haemophilus influenzae; and iii) estimate the rates of transformation in Haemophilus influenzae isolated from clinical sources. (3) Phage studies: In an effort to test hypotheses about the conditions for the maintenance of bacteriophage and their co-evolution of these viruses and their hosts, we will examine the population dynamics of lytic and temperate phage and E. coli in the enteric and fecal flora of mice. These investigations will include: i) the development and analysis of mathematical models, ii) studies with experimental populations, iii) molecular assays for changes in the structure of plasmid and the movement of transposons. While this research is motivated primarily by its implications for evolutionary biology, the problems being considered are of direct concern to the health science. Most clinically important antibiotic resistance and some bacterial pathogenesis are coded for by genes carried on transposon, plasmids and prophage. The ability to identify pathogenic strains of bacteria by typing methods is contingent on the stability of bacteria genotypes, which, in turn depends on the amount of recombination between clones.